1. Field of the Invention
This invention generally relates to thermal printers for printing desired characters and images on heat-sensitive paper directly or on plain paper through heat-resistive ribbon using a thermal print head, and, in particular, to a system for driving such a thermal print head.
2. Description of the Prior Art
Thermal printers are well known in the art. Typically, a thermal printer includes a thermal print head provided with a plurality of heat-producing elements such as electrically resistive elements arranged in a single array at a predetermined pitch and a driving circuit to supply driving current pulses to the array of heat-producing elements selectively in accordance with an image signal supplied thereto. A sheet of heat-sensitive paper is moved with respect to and in contact with the thermal print head so that desired portions of the paper are "burned" or darkened thereby forming a reproduced image in the form of dot matrix.
There has been recently developed a direct-drive type thermal printer in which a driving circuit for driving the thermal print head, which generally includes switching transistors each connected to the corresponding one of the heat-producing elements and which is fabricated in the form of an IC, is mounted integrally and directly on the thermal print head. Such a driving circuit typically includes serial-to-parallel shift registers which serially receive image data for a single line and then supply the thus received image data to the heat-producing elements in parallel. In such a structure, latches are commonly provided between the shift registers and the heat-producing elements in order to increase operational speed.
It is true that various advantages may be obtained by using such a direct-drive type thermal printer. However, it is also true that there are some areas which need to be further refined and which need to be improved in order to obviate some disadvantages which are inherent in the direct-drive type thermal printer. For example, in order to obtain a digital image signal which may be applied to the thermal print head for carrying out thermal printing operation, analog image information must first be converted into digital image data with the use of an analog-to-digital (A/D) converter by scanning an original.
In such thermal printers, use is commonly made of a fixed sampling rate so as to allow to carry out a high-speed printing operation. With such a structure, however, as shown in FIG. 1, although no problem arises as long as the amplitude variation of input analog signal is rather small as indicated by a portion A, in which the dot density is reasonably high to allow easy recognition of a reproduced image, a poor reproduced image will result if it has a larger amplitude variation as indicated by a portion B, in which the dot density is too low and thus it is difficult for a viewer to recognize a reproduced image. Accordingly, a simple increase in printing speed with a fixed sampling rate will produce a reproduced image of irregularly varying image densities since the dot density will vary depending upon the amplitude variation of analog image signal obtained by scanning an original. Other disadvantages will also loom large when a high-speed printing operation is desired in such thermal printers which need to be obviated as will become clear as one reads through this specification. Moreover, it is also important to devise a means for prolonging the service life of a thermal print head because it can easily lead to damages or malfunctioning due to repetitive application of heat.